Detection of liquids

ABSTRACT

Liquid, particularly leaked oil or other hydrophobic liquid, is detected by a sensor. This preferably has a hydrophobic membrane (G) that takes up the oil selectively, a radiation source (A) that beams radiation at an interface between the membrane (G) and a window (F), and a radiation detector (H) that receives radiation resulting from interaction (such as reflection, scattering or fluorescence) of the input radiation with the liquid-containing membrane (G). The detector may employ a spectrofluorimeter (H) whose output can be used to characterise the liquid.

TECHNICAL FIELD

The present invention relates to a method and apparatus for use in thedetection of liquids, particularly hydrophobic liquids such as oils. Itmay be used for monitoring for leakage. A preferred type of embodimentemploys an optical method that is capable of detecting and preferablyidentifying hydrocarbon liquids such as mineral or synthetic oils,petroleum, diesel, insulating oils etc. emanating from a leaking vessel,container or storage device.

A number of vessels are routinely used to carry or hold hydrocarbonsolvents such as oils in a wide variety of applications. Examplesinclude oil filled underground power cables, underground petroleumstorage tanks, above surface oil containers for industrial or domesticheating, oil filled power transformers and equipment etc. Release of thecontainer's contents, deliberate or accidental or via corrosion overtime will have economic and environmental consequences. The routinemonitoring of the release of oil/solvents from these containers can bean arduous task because of the myriad of locations, distribution andvarieties of such devices. A more convenient method would be one where asensor device is placed at each container location and was able toperform continues or periodic monitoring as required. Ideally, thesensor should be able to detect any oil/solvent spillage as soon as itoccurs in order that remedial actions can be made to minimise anyongoing loss into the environment. Such a device is referred to here asan in-situ sensor or monitor. Ideally, a remotely controlled sensordevice that could automatically warn of oil/solvent leakage is preferredespecially where container vessels or locations are difficult to access,are widely distributed or where checks for leakage are madeinfrequently. Examples of such situations are discussed below.

Oil Filled Underground Power Cables

The continual supply of electrical power throughout a country relies onthe integrity of underground and overhead power cables. Ease ofmaintenance requires that most power cables are run above ground, butwhere this is not possible, such as in cities, the power lines areburied some meters underground. At the operating voltages of 132 kV andabove, many of the cables in service are of the oil filled type.Oil-filled cables are normally laid in sections of between 200-400meters, which are then joined together in specially constructed jointbays. The cables and joints are then encased in a special backfillmaterial such as speciality grade sands or cement-bound-sand (CBS).

Voids in the cable insulation can result in partial discharge activityand ultimately electrical breakdown of the cable. In an oil filled cablethe oil, if maintained under sufficient pressure, prevents the formationof gaseous voids. The hydraulic system is designed to be maintained at apositive pressure at the highest points on the route profile and forthis maximum static pressure at the lower points on the profile can beup to 5.25 bar.

Problems arise with this type of cable when leaks appear in the pressureretaining metal jacket. Where there is a leak, the cable must beswitched out if adequate pressure cannot be maintained in order toprevent the risk of electrical breakdown of the cable. The oil used innew cable installations is a synthetic mixture of alkylated benzenes,with the greatest component being dodecylbenzene (DDB). (Older cableinstallations employ mineral oil for insulation, but this is graduallybeing phased out.) Although it has not been shown to be directlycarcinogenic to humans, it is of a class of chemicals (substitutedbenzenes) some of whom do have toxic properties, so there areenvironmental implications associated with leakage of this oil.

Due to the nature of the cable, leaks most often occur where two cableends are joined, in specially constructed joint bays. Leaks in the bodyof the cable are much rarer, and are usually only caused by theover-zealous use of earth digging machinery, and so are usually locatedimmediately. Nevertheless, should a leak occur it would be detected by afall in operating pressure over a period of time. Then the problem liesin locating in which of the many joint bays along the length of thecable the leak has sprung. Prior to this invention, detection relied onhydraulic bridge techniques, which are both time consuming andunreliable. A much-preferred method would be to install in each jointbay a device capable of providing immediate notification of a leakcondition and thus ensure swift remedial measures, avoid the risk ofexcavating a healthy bay and minimise any disruption to power supplies.

Underground Petrol Tanks

The burial of tanks containing hydrocarbon liquids has been a method ofstorage around the world. One of the main reasons that this method isemployed is for the reduction in the risk of fire and explosion that isafforded. When sited underground the tank is protected from damage bythe myriad of possible causes, and will also save on space. However,placing tanks under the ground has its own hazards. The particularproblem with underground tank storage is one of tank corrosion. Whereasabove ground tanks are easily inspected, underground tanks by the natureof their position are a more difficult monitoring challenge. Thestability of the soil is not easily assessed, any leaks that may beoccurring may continue for months or even years, and even a small leakof one drop per second may result in a loss to the soil of 400 litersper annum. Awareness of the problems has been increasing; in Britain,particularly with the Environmental Protection Act of 1990 and theEnvironment Act of 1995 emphasising the polluter pays principle, andincreased concern over water supplies. In the USA, awareness and concernare particularly high, especially in some areas where dependence ongroundwater is high. The Environmental Protection Agency estimates that41,600,000 liters of petrol alone may be leaking from undergroundstorage tanks every year.

A preferred route to monitoring possible leaks from underground tankswould be to install an in-situ sensor device that was buried near thetank and capable of sensing whether or not a leak had occurred, thusallowing the user to determine the integrity of the vessel on acontinuous basis.

IN-SITU MONITORING DEVICES—PRIOR ART

One design for such a sensor (TraceTek from Raychem, USA) (seehttp://www.raychem.com/products/chemelex/technolocy.htm for details.)involves the two poles of an electrical switch being separated by adegradable polymer. Not only is this system expensive and difficult toinstall it can also produce false indications where there are low levelbackground traces of oil, as this tends to degrade the polymer overextended periods of time.

Other methods of leak detection involve hydraulic bridge techniques.This system requires very small pressure differences to be measured andthese measurements can be difficult where there are transient pressurevariations and/or where cable records are unreliable and/or where thereare localised thermal conditions owing for example to another heatsource.

DISCLOSURE OF INVENTION

According to the present invention in a first aspect there is provided amethod of monitoring for the presence of liquid at a site comprising:locating at said site a sensor assembly comprising a radiation sourceand a radiation detector and/or analyser arranged to detect and/oranalyse radiation which results from the emission of radiation by thesource; causing the radiation source to irradiate a sensing location;and employing said detector/analyser to receive radiation, thearrangement being such that the nature and/or amount of radiationreceived by the detector/analyser is affected by the presence of liquidat the sensing location. The liquid may be a hydrophobic liquid such asoil. The sensor assembly may include a hydrophobic membrane or otherelement which preferentially takes up hydrophobic liquid. This affectsits optical properties, e.g. reflectance of light at a membrane/glassinterface. The element may be or include a fluorocarbon, e.g.polyvinylidene fluoride.

Radiation from the source interacts with the liquid in the sensinglocation, e.g. by one or more of reflection, absorption, transmission,scattering and fluorescence. Radiation resulting from the interaction isdetected and/or analysed by the detector/analyser.

In a second aspect the invention provides an assembly comprising avessel containing a liquid and a sensor assembly located at a sitepotentially contaminated by liquid leaking from the vessel and adaptedto carry out the method as defined above.

In a third aspect the invention provides a sensor assembly for use inmonitoring for the presence of hydrophobic liquid at a site, saidassembly comprising: a hydrophobic element which is disposed so that inuse it is exposed to the environment at the site and which is adapted totake up hydrophobic liquid; a radiation source arranged to irradiate atleast a portion of the hydrophobic element; and a radiation detectorand/or analyser arranged to receive radiation resulting from theinteraction of the source's radiation with the hydrophobic element.

In a preferred type of embodiment the invention provides an in-situdevice for the detection and identification of oil or other hydrocarbonproducts that have leaked from vessels such as underground power cablesor petrol storage tanks.

Detection of oil may be achieved by measuring the intensity of lightreflected or emitted from a hydrophobic membrane at an optical window incontact with the external environment, when oil is present in theenvironment, it is absorbed into the hydrophobic membrane causing achange in the intensity of the reflected beam. The membrane can be onefrom the fluorocarbon range of membrane materials such as polyvinylidenefluoride. Identification of the oil is provided by measuring thespectral properties of the reflected beam or the spectral properties oflight emanating from oil absorbed in the membrane.

In a preferred embodiment, the sensor has been designed for specificapplication to detecting oil leakage from underground power cablesalthough it has clear application in other situations where oil or otherhydrocarbons may leak from a vessel located underground, above ground orin water.

There may be a plurality of sensors for installation at differentlocations around a potential source of liquid. They may be connected viawaveguides (such as fiber optics) to a detector/analyser. There may be a“multiplexing unit” such that the detector/analyser is connectable toone sensor at a time, the connected sensor being selectable and/ordetermined by programmed switching.

The invention can be left to operate in-situ at the monitoring siteenabling convenient continuous or periodic monitoring of theenvironment. Using telecommunications methods known to those skilled inthe art, it is possible to transfer data from the monitoring site to aremote destination. The invention has several applications wheremonitoring oil or hydrocarbon leaks is required or preferred. Examplesare shown in the technical description.

In the preferred embodiment, the device can be buried in sand or soil orimmersed in water that surrounds an oil carrying vessel or container. Ifoil leaks from the vessel and contacts the sensor, it will be detectedas a change in signal intensity or spectral characteristics. Such asystem can therefore be used as an in-situ monitoring device that istriggered when a leak has occurred.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of apparatus used to test prototypetransducers;

FIG. 2 is a circuit diagram of a prototype reflectance measurementcircuit as used with the apparatus of FIG. 1;

FIGS. 3-6 are graphs of reflectance voltage (V) against time(s) showingresponses of the prototype transducer;

FIG. 7 is a view like FIG. 2 showing a modified circuit diagram;

FIG. 8 is a schematic view of apparatus used to evaluate a sensor;

FIGS. 9 and 10 are graphs like FIGS. 3-6 showing the response of theapparatus of FIG. 8;

FIG. 11 is a schematic view of a second form of sensor assembly in use;

FIGS. 12A, B, C and D are fluorescence spectra produced using theassembly of FIG. 11; and

FIG. 13 is a schematic view of a multi-sensor device, with one sensorunit shown expanded.

MODES FOR CARRYING OUT THE INVENTION

1. Development of Sensor Device for Monitoring Cable Oil Leaks

One of the simplest optical measurement techniques was chosen for thesensor: the measurement of the intensity of the beam reflected from asand surface adjacent a window. One or a number of wavelengths of lightcould be monitored. The presence of oil in the sand directly against aglass sensing window should cause a significant drop in the beam'sreflected intensity due to absorption by the oil and diffractionoccurring at the glass/oil interface. Interference in the optical signalwould be caused mainly by the presence of water in the surroundingmedium (which may be cement bound sand (CBS)), which to some extentwould mimic the properties of oil in the, sand.

Initial investigations of the optical properties of known mixtures ofsand, water and cable oil were performed using an Instruments SAFluoromax II spectrofluorimeter operating in front-face collection mode,with the sample contained within methacrylate UV fluorimetry cuvettes,the total reflectance of each of the standard mixtures was measured fordifferent excitation wavelengths.

Five prototype sensors were built in-house for sensor characterisation.Solid state components were selected for their longevity, reliabilityand very low power consumption.

The apparatus is shown in FIG. 1. Its design was selected for itssimplicity and ease of testing. It has a housing 8, one side of whichhas a glass window 10. Within the housing there is a near-IR photodiodesource 12 which illuminates a surface of the window 10, which forms thesensing area of the module to be brought into contact with the sandsamples 14, contained in a Petri dish 18. Light reflected from thesample window 10 is detected and measured using a near-IRphototransistor 16, also mounted in the housing. Controlled amounts ofoil can be added (20) to the sand 14.

The schematic for the reflectance measurement electronics is shown inFIG. 2. A photodiode D1 (e.g. SFH 409) and a first resistor R1 (e.g. 100ohm) are in parallel with a phototransistor T1 (e.g. SD 3443) and asecond resistor R2 (e.g. 1 k.ohm), across a DC voltage (e.g. 5 v).Current through the phototransistor, T1, is dependent on the flux oflight illuminating the transistor's base electrode. The limitingresistor, R2, converts the current to an output voltage between zero andfive volts.

Infrared components were chosen so that stray light would not interferewith the results (this was particularly important for tests carried outin the laboratory), and for optimum sensitivity as silicon operates mostefficiently in the near IR. For simplicity, no filters were used and thetotal LED output was used as the excitation signal. Other sources givingrise to other wavelengths could also be used if required for aparticular application. Absorption of such wavelengths could be used forthe sensing mechanism especially where spectroscopic measurements(source emission or detection over a series of wavelengths) are beingmade. Alternatively, the fluorescence or Raman characteristics of theoil could also be measured using an alternative optical set-up. Thephotodiode, D1, and phototransistor, T1, were chosen for theirsimilarities in spectral output and response.

In a further embodiment of the sensor, light from the source could bechannelled along a fiber optic tube or planar waveguide. In this case,the returned light would be modified in the presence of oil. The fiberoptic design is particularly relevant when distributed sensing isrequired as a number of fibres could be multiplexed to one sensor tocover a larger sampling area.

1.1 Verification of the Prototype Transducer

To represent the CBS that surrounds the underground cables, test sampleswere made by mixing dried soft building sand with water. Water is mostoften present in the CBS between 0% and 10% by mass. Saturation, whichprevents oil from entering the sand, was found to occur for watercontents approaching 30% so a maximum of 20% water was used duringtesting. The sand and water mixture (totalling approximately 20 g) wasthen placed inside a plastic petri-dish which was placed upon the glasswindow of the sensor (as in FIG. 1). Cable oil was then added dropwiseto the sand mixture to simulate oil from a leaking cable encroachingupon the active sensing region. This arrangement was chosen as itpermits rapid evaluation of the sensor (the oil is drawn towards thesensing window by gravity and capillary action) and it requires aminimal amount of sand and oil, so results in a minimum of wastematerial.

The response of the transducer to oil additions is shown in FIG. 3. Oilwas added to sand containing 10% water (by weight). Each arrowrepresents addition of 1% by weight of oil. There is clearly a drop inreflectance proportional to the amount of oil added, raising thepossibility of a quantitative oil sensor. It is interesting to note therapid sensor response time observed in the figure. It takes less thanfive seconds for the reflectance voltage to stabilise at a new valueafter oil addition.

Because water content in the sand may exhibit wide variations, it wasimportant to determine the extent to which this may interfere with oilmeasurement. Unfortunately, it was found to have a very considerableeffect. FIG. 4 shows the sensor output voltage (which is proportional toreflectance) as water is added to sand. Initially the sand contained 10%water and each arrow represents addition of 1% by weight of water. Theresponse to water is similar to that for oil shown in FIG. 3,effectively prohibiting the use of this method for oil detection whenwater content may also vary, as there is no way to differentiate betweenwater and oil at the sensing window. Monitoring rate of change insteadof absolute reflectance could be used to detect a flood of oil, sincethis would give a sudden change in response whereas the passage of waterthrough the sand would be more gradual. However, we decided to develop aphysical solution to the problem.

1.2 Improvement in Selectivity and Sensitivity: Use of Oil-SelectiveMembrane

To remove the problem of water interference, a Fluorotrans membrane(polyvinylidene fluoride) was introduced between the sand and thesensing window. (It was placed in the petri dish 18 before the sand 14was added.) This membrane is extremely hydrophobic, repelling water fromthe sensing surface while attracting organic fluids such as oil. Inaddition to increasing selectivity, this also increased sensitivity, asthe change in reflectance of the membrane as it absorbs oil isconsiderably larger than that directly observed in sand.

FIG. 5 shows the response obtained of the membrane-covered sensor whenoil was added to the sand, each arrow indicating a 1% increase in oilconcentration. The response time is significantly increased for lowconcentrations with respect to that achieved without the membrane, butthe latency is still small with respect to the 24 hour sampling periodanticipated. FIG. 6 shows how insensitive the sensor response is tochanges in water concentration, each arrow representing a 1% increase inwater concentration.

1.3 Construction of Prototype

Having proved the concept of the oil detection method, the method wasembodied into a manufacturable sensor suitable for long term operationunderground. A tube like design was used that incorporated the sensor atone end (termed the sensor head) (FIG. 8). The sensor head 30 comprisesthe oil detecting assembly, consisting of the reflectance measurementcircuit, a glass window, a disc of Fluorotrans membrane and a removableglass retaining ring 32 used to keep the glass and membrane discs inplace.

The reflectance measurement circuit used in this device is shown in FIG.7. This is generally as shown in FIG. 2 and described above, except that(i) the current requirement of the circuit was reduced from 50 mA to 2mA by altering the value of resistor R1 to 2 k.ohm (this results in adifference in reflectance voltage with respect to that provided by theprototype sensors); and (ii) a trimmer R3 was added in series with R1 toenable the sensitivity of the transducer to be adjusted to account forany manufacturing-induced variations in the performance of thephotodiode and phototransistor. The revised circuit is shown in FIG. 7.The trimmer R3 may be a 100 k.ohm multi-turn cermet trimmer.

The sensor device was evaluated using the experimental set-up shown inFIG. 8. The large size of the device necessitated that evaluation wascarried out in a much larger amount of sand 34 (500 g) than that usedwhen testing prototypes. This, coupled with the inverted nature of thetransducer, resulted in slow transportation of oil to the sensing headand therefore lengthened the response time of the sensor considerably.Typically oil added to the sand surface took between 15 and 30 minutesto reach the sensing membrane. The response observed when the oilreaches the sensor surface is shown in FIG. 9. Oil was added 15 minutesprior to the start of the plot.

As a precautionary measure, it was decided that a protective mesh shouldbe added in front of the sensing membrane to prevent the possibility ofsand particles tearing the membrane when the transducer is beingpositioned. Due to the reflective nature of the mesh, the response tooil is slightly different (see FIG. 10; oil added 15 minutes prior tostart of plot) but the distinction between the presence and absence ofoil is still very clear.

1.3.1 Long Term Stability Tests

Additional tests were conducted to determine the integrity of the newsensor head in water and sand. These involved:

1. Leaving two transducers submerged in water for one month and checkingthat water did not enter the assembly or in any way affect the behaviourof the transducer.

2. Leaving two transducers buried in a beaker of sand containing 10%water for one month, then adding oil to check if the characteristic oilresponse was still observed.

In both cases both transducers passed without problems.

Identification of Different Hydrocarbons

In order to demonstrate the detection of different types of hydrocarbonproducts, the sensor design was modified in order to measure thespectral characteristics of oils that were absorbed into the membranefrom the environment. This was simulated in the laboratory by using afibre optic configuration that illuminated the membrane in contact withsoil and collecting the emitted light. In the presence of oil in themembrane, fluorescence emission occurs which is characteristic of eachtype of hydrocarbon material. The apparatus is shown in FIG. 11. Theinner surface of a quartz cuvette F was lined with the fluoropolymerhydrophobic membrane G. The quartz-membrane interface represents theoptical window design employed in the technical description above. Thecuvette was packed with sand J to simulate the arrangement theoptical-fibre sensor would take in the environment.

The optical fibre was connected to a spectrofluorimeter instrument H andsynchronous scans of different fuel and oil samples were taken using theoptical fibre collection system. The arrangement is shown in FIG. 11.The letters in this figure refer to the following: A=lamp,B=monochromator, C=focussing lens, D=excitation fibre, E=emission fibre,F=cuvette full of sand, G=membrane.

The fluorescence excitation beam was focussed onto the membrane using aquartz lens on the end of a 25-stranded quartz fibre-optic bundle.Fluorescence collection was facilitated by another 25-stranded group offibres co-bundled with the excitation fibres. Synchronous scans wereperformed between 250 nm and 500 nm. FIG. 12 shows the fluoroescencespectra obtained from 4 different oil samples: cable oil (FIG. 12A),transformer oil (FIG. 12B), petrol (FIG. 12C) and diesel (FIG. 12D).This experimental evidence clearly shows the sensor can be used for theidentification of different oils and other hydrocarbon liquids such aspetroleum and diesel.

FIG. 13 shows a multi-point fibre optic sensor using the same principleof detection. A multiplicity (e.g. 10) of sensors C are each connectedto a common control unit G by a respective optical cable B containingtwo sets of fibres—one set for transmission of light to the membrane E,the other set K for collection of light reflected or emitted from the(liquid hydrocarbons on the) membrane. The transmission and collectionfibres together are referred to as a pair.

Light is channelled down a transmission fibre optic bundle J whichterminates in a sensor probe head C. The sensor head employs anidentical membrane to that used previously, which is illuminated by thelight from the transmission optical fibres. Light reflected from themembrane surface is collected by collection fibres K in the bundle andis delivered to a detector H.

When oil is present, the intensity of the reflected light diminishes orthe spectroscopic properties of the emitted light is modified owing tothe presence of liquid hydrocarbons. The length of the fibre optic paircan vary (typically 1-10 m in length).

A number of fibre optic pairs can be integrated by means of a singlecontrol device G. The device controls which fibre optic pair will beoperated. Typically 1-10 pairs are used, and pairs may be of differentlength.

The control device is controlled by a microprocessor and can becontrolled remotely using appropriate telecommunications.

What is claimed is:
 1. A method of monitoring for the presence ofhydrophobic liquid at a site comprising: locating at said site a sensorassembly which comprises a polyvinylidene fluoride membrane which isadapted to take up hydrophobic liquid from the site, radiation inputmeans connected to a radiation source and arranged to irradiate saidmembrane, and radiation output means connected to a radiation detectorand/or analyser arranged to detect and/or analyse radiation whichresults from the irradiation of said membrane by said radiation inputmeans; causing the radiation input means to irradiate said membrane; andemploying said detector/analyser to receive radiation via said radiationoutput means, the arrangement being such that the nature and/or amountof radiation received by the detector/analyser is affected by thepresence of liquid at the site.
 2. A method according to claim 1 whereinthe radiation source and input means are operated to direct radiationtowards said membrane and the detector/analyser and output means areused to receive radiation reflected from the membrane.
 3. A methodaccording to claim 1 wherein the radiation source and input means areoperated to direct radiation towards said membrane and thedetector/analyser and output means are used to receive radiationscattered from said membrane.
 4. A method according to claim 1 whereinthe radiation source and input means are operated to direct radiationtowards said membrane and the detector/analyser and output means areused to receive radiation transmitted through said membrane.
 5. A methodaccording to claim 1 including a step of examining the spectroscopiccharacteristics of the radiation received by the detector/analyser toprovide data relating to the chemical nature of liquid at the site.
 6. Amethod according to claim 1 wherein the radiation source and thedetector/analyser are remote from the site and are connected to theinput and output means, respectively, via waveguide means.
 7. A methodaccording to claim 1 wherein there are a plurality of sensor assemblieswhich are located at different sites, and the method includes switchingthe connection of the radiation source and/or the detector/analyserbetween different sensor assemblies.
 8. A sensor assembly for use inmonitoring for the presence of hydrophobic liquid at a site, saidassembly comprising: a hydrophobic element comprising a polyvinylidenefluoride membrane which is disposed so that in use it is exposed to theenvironment at a sensing location and which is adapted to take uphydrophobic liquid; a radiation source arranged to irradiate at least aportion of the hydrophobic element; and a radiation detector and/oranalyser arranged to receive radiation resulting from the interaction ofthe source's radiation with the hydrophobic element.
 9. A sensorassembly according to claim 8 which includes a housing containing, orcoupled to, said radiation source and said radiation detector and/oranalyser; said housing having window means confronting said hydrophobicelement; and said radiation source and detector/analyser being disposedor coupled so that radiation from the source can pass outwardly throughthe window means, and undergo reflection and/or other interaction at thehydrophobic element, interacted radiation passing inwardly through thewindow means to reach the detector analyser.
 10. An assembly accordingto claim 6 wherein the detector/analyser comprises means forspectroscopic analysis.
 11. An assembly according to claim 8 furthercomprising a vessel containing a hydrophobic liquid and wherein saidhydrophobic element is located at a site potentially contaminated byliquid leaking from the vessel whereby said sensor assembly is operableto detect leakage of the liquid.
 12. An assembly according to claim 8adapted to carry out remote monitoring by means of a telecommunicationlink arranged to transfer data from the sensor assembly to a remotedestination.
 13. An assembly according to claim 8 wherein the radiationsource and detector/analyser are adapted to be remote from the sensinglocation, being coupled to waveguide means for conveying radiation toand from the sensing location.
 14. An assembly for carrying out themethod of claim 7 and comprising a detector/analyser and/or a radiationsource connected to a switching unit which is connected to a pluralityof sensor assemblies and is operable to switch the connection of theradiation source and/or the detector/analyser between different sensorassemblies.